Paraffin mixtures occur commonly both in nature and in a refinery. They are present in gas recovery at the oil field itself. LPG, for example, is a mixture of principally C3 and C4 compounds. Field condensate is a mixture of C4 and C5 paraffins that is condensed "in the oil field" as the gas and oil are separated prior to sending the gas and liquid products to separate transportation systems. Paraffin mixtures also occur in refinery separation systems depending upon the degree of separation employed. It is common to find mixed C2/C3 streams within the FCC gas plant for example.
It is desirable on many occasions to further process these mixtures to valuable products including olefins. There are essentially two routes available to react paraffins to olefins and these routes compete with each other. Paraffins can be cracked thermally to produce a mixture that consists of essentially olefins of lower carbon number than the feed, except in the case of ethane where the principal product of thermal dehydrogenation is ethylene. Processing typically occurs at high temperatures and short residence times.
Paraffins can also be reacted catalytically. In these processes, specific catalysts are chosen to allow for the dehydrogenation of the paraffin feed to the olefin of the same carbon number. Processing typically occurs at significantly lower temperatures and longer residence times than thermal cracking (to minimize thermal reactions) and the once through yield of the olefin is strongly influenced by thermodynamic equilibrium.
The products one would achieve by thermal cracking a mixture of paraffins are drastically different from the products one would achieve from catalytically processing the same mixture.
Furthermore, there are significant differences between various paraffins which become important during processing of mixtures. Using propane and ethane as an example, propane is a less refractory molecule (easier to react) than ethane. Attached as FIG. 1 is a plot of the reaction velocity constants for light paraffins as a function of temperature. Note that at any temperature propane will react between two (2) and five (5) times more rapidly than ethane. Attached as FIG. 2 is a plot of the equilibrium curves for the various paraffins. One can see that at any given temperature the propylene/propane ratio will be much higher than the ethylene/ethane ratio, indicating a more favorable equilibrium for propane.
Catalytic dehydrogenation of propane yields propylene. Propane can also be thermally cracked into C2+C1, and propylene product can be cracked or polymerized. These are the degradation reactions. Ethane on the other hand principally dehydrogenates, either thermally or catalytically, but requires more severe conditions in terms of temperature. There is relatively little cracking, i.e., into C1+C1, and little polymerization, i.e., into C4+.
For example, at 1100.degree. F., the thermodynamic equilibrium for propane gives 40% olefins and 60% paraffins while for ethane at the same temperature there is only 14% olefins. Thermal reactions for these paraffins are essentially nonexistent at 1100.degree. F. If, however, the temperature is raised to 1400.degree. F., equilibrium predicts 85% olefins for propylene/propane and 53% for ethylene/ethane. At these temperatures, thermal reactions are significant. At a 1.0 second residence time, 60% thermal conversion would occur for a pure propane stream while at a 0.25 second residence time, only 20% conversion would occur for the same stream. Similarly, 20% thermal conversion of ethane would occur in 1.0 second but just 5% in 0.25 seconds. Therefore, processing a mixture of paraffins in a catalytic dehydrogenation unit leads to feeds achieving considerably different conversions. Within the constraints of equilibrium, it is not possible to simultaneously catalytically dehydrogenate a mixture economically.
This analogy also holds for other light hydrocarbons. For example, with mixtures of propane and butane, butane is easier to react and will have a more favorable equilibrium than propane. The differences in reactivity and equilibrium cause problems in attempts to dehydrogenate paraffin mixtures in "conventional" equipment. Problems are even found in conventional processes when mixed C4 streams are to be processed wherein kinetic and thermodynamic equilibrium differences between isobutane and normal butane exist.
With respect to the yields desired, the prior art provides two options for the processing of paraffin mixtures. If thermal products are desired, the feeds are cracked (in a pyrolysis coil for example) either together or separately. This option maximizes ethylene in the case of ethane/propane feed or mixed products for higher hydrocarbon feeds. For the most part, conversions are high and recycles are low. The exception, however, is ethane For ethane pyrolysis, conversions are limited to 60-70% of the reactor feed in order to minimize fouling. Therefore, in order to fully process an ethane stream, considerable recycle capability must be incorporated.
A pyrolysis plant designed for a mixture of paraffins with carbon number higher than 2, e.g. propane/butane, would have limited recycle capability and thus would have limited capacity to process ethane. This is important in considering the range of potential concentrations of ethane in mixtures as well. If the plant is designed for a 20% ethane/80% propane stream, it would have limited capacity to process an 80/20 ethane/propane stream. The mass flow to the plant, and therefore capacity, would have to be reduced in order to accommodate recycle.
Consider the same ethane/propane mixture, however, with catalytic yields (high propylene in this case) desired. Two separate trains of conventional processing are required. The mixed feed would first require separation of the paraffins, then individual processing of the paraffins, possibly including product separation, and finally combination of the effluents. Ethane cannot be processed economically in a fixed bed catalytic dehydrogenation process since conversions would be on the order of 20% and thus would result in very high recycle rates. This would require substantial separation system energy and capacity, substantially increasing cost. For the ethane/propane mixture of the example, a combination of ethane cracking coils and propane dehydrogenation units are required. For mixtures of paraffins with a higher carbon number, separate dehydrogenation units would probably be required. It is also true that once a commitment is made to proceed by dehydrogenation, there is no option for cracking.
The differences in conversion at similar operating conditions creates problems when coprocessing mixtures. Here, it should be noted that co-processing two feeds in the same reactor at the same time is distinct from co-processing feeds in parallel reactors, each of which operates at separate conditions. Since only one set of conditions (temperature, pressure, residence time, catalyst type, etc.) can exist in a single reactor at any one time, both feeds see that set of conditions. This set may or may not be optimal for one but given the difference in kinetics and equilibrium, will certainly not be optimal for both.
In order to co-process two hydrocarbons in the same piece of equipment, one feed must be overreacted and the other underreacted, or the lighter one must be reacted to an economic level and the heavier one severely overreacted leading to a rapid catalyst fouling. To react the heavier feed optimally would result in only small conversion levels for the light feed which would represent a significant underutilization of the equipment, and generate huge recycles within the plant.
Of the possible options for the known methods set forth above, there is really no practical option. Operation at economically acceptable conversions of the heavier feed underutilizes the light feed and operation at economically acceptable conversions for the light feed results in rapid catalyst fouling due to overreaction of the heavy feed. There may be some exceptions to this when processing mixtures with very high concentrations of the heavier feed component.
In fixed bed processes, when trying to operate the short times preferred for dehydrogenation, a shallow bed is required and typically a radial flow bed is used. In a radial flow bed, solids are located in a thin annulus, vertically oriented within the bed. The feed is introduced in a complicated internal distribution system over the bed and then passes through the thin annulus. By fixing the solids in the annulus, the maximum surface area for flow is attained in a minimum vessel diameter. Flow area determines capacity since gas velocity must be low to avoid high pressure drops. There is no possibility for heat addition and/or removal from such a reactor. Heat for the endothermic dehydrogenation reaction is provided by preheating the feed which contains significant quantities of diluent acting as a heat carrier.
Tubular reactors can provide some heat addition and/or removal but are very limited in their capacity to do so. In order to get short residence times, catalyst depths (or tube lengths in this case) must be limited given the low gas velocity required to avoid high pressure drop, due to the detrimental effect of absolute pressure on conversion (equilibrium limit). In order to avoid hot tube walls and excessive reaction at the wall, the temperature driving forces are held low. Thus, the total heat that can be transferred through the wall of the tube is low since it is proportional to the tube surface area and temperature difference. In order to satisfy even minimal heat removal or addition requirements, a great many small diameter tubes having high surface to flow volume in each tube are required, leading to an extremely complex distribution system into and out of the tube banks.
The only option for dehydrogenating paraffin mixtures using conventional processing equipment is to run separate reaction systems in parallel. This however requires separate feed preparation and product recovery equipment for each feed component, adding significantly to the cost of such a unit.